The present invention relates to a collimator, a radiological imaging apparatus and a nuclear medicine diagnosis apparatus.
A radiological imaging apparatus has been known in which medicines given a token for discrimination by a radioisotope (hereinafter referred to as RI) are dosed to the intra-body of an object to be examined, gamma rays radiated from the RI are measured and the distribution of the medicines in the examining object body is imaged.
A known radiological imaging apparatus has single crystals of sodium iodide (hereinafter abbreviated as NaI) for converting gamma rays to optical rays and a photomultiplier tube for converting the optical rays the NaI emits to an electric signal, additionally having a succeeding stage of an electric circuit by which the position of an incident gamma ray is determined.
The RI dosed to the intra-body of examining object radiates gamma rays in all-around directions and therefore, for the sake of imaging, a collimator for permitting only gamma rays in a specified direction to be transmitted is used (see JP-A-11-30670, for example).
A general collimator as viewed from an examining object is schematically illustrated in FIG. 8A, the collimator designated by reference numeral 70 being depicted as having a honeycomb structure of a thin lead plate forming many hexagonal holes 71 (see FIG. 8B). The performance of this type of collimator 70 can be evaluated by its sensitivity indicative of the ability to transmit how many of gamma rays radiated from the examining object not shown and its resolution indicative of the ability to obtain images which are determined as to how far they are detailed. The sensitivity referred to herein can be increased by, for example, decreasing the thickness of the lead plate of honeycomb structure (hereinafter referred to as a partition wall thickness), by forming the hole 71 largely and by decreasing the thickness of the whole honeycomb structure (hereinafter referred to as collimator thickness). On the other hand, the resolution can be increased by making the hole 71 small or by increasing the collimator thickness.
Typically, the collimator 70 can be produced through various methods as described below.
In a method shown in FIG. 9, a lead plate 73 is wound around a base member 72 of aluminum, for example, and the lead plate 73 integral with the base member 72 is rolled in a hexagonal form, thus forming a thin or minute strand 74. Then, a plurality of strands 74 are put and bonded together and thereafter respective base members 72 are resolved in an alkaline solution, for example, so as to be removed, so that the collimator 70 of honeycomb structure having many hexagonal holes 71 can eventually be produced.
In another method shown in FIG. 10, molds for forming hexagonal holes (hereinafter referred to as pins 76) are used, the pins 76 being formed by the same number as that of holes the collimator has. Opposite ends of a plurality of pins 76 arranged at predetermined intervals are held in position by means of meshed plates not shown, leaving behind gaps into which molten lead is poured and after the poured lead is cooled, the plurality of pins 76 are drawn out to thereby produce a collimator of honeycomb structure.
Incidentally, as a substitution for the NaI, a semiconductor material having high energy resolution has recently been available and a radiological detection device using a plurality of detectors each made of the semiconductor material has been put into practice.
Being different from the NaI, the radiological detection device has the function to directly convert incident gamma rays to electric signals. Therefore, the radiological detection device has an advantage that the number of conversion operations can be reduced as compared to the NaI combined with the photomultiplier tube for conversion to optical light which in turn is converted to an electric signal as described previously and so the energy utilization efficiency can be improved and noise can be reduced to enable the high energy resolution to be obtained.
Typically, this type of radiological detection device is so structured that detectors are arranged at the same pitch as the size of a matrix to be detected, with each detector having an easy-to-produce rectangular parallelepiped form and being arranged while having a square surface opposing the examining object and having its longitudinal direction aligned to the direction in which gamma rays are detected.